Handheld power tools, including reciprocating saws, are used by professionals in many industries, including construction, manufacturing, and outdoor landscaping, on a regular basis. During operation of the handheld tool, various vibrational forces are generated in the tool and some of the vibrations are transmitted to the hands and body of the operator. Some vibrations are regular, such as vibrations from a reciprocating motor and saw blade that repeat at a regular interval corresponding to the operation of the saw. Other vibrations occur randomly, such as those vibrations generated by interactions between a saw blade and a work piece or those produced by a footpad through which the reciprocating tool moves and the work piece as the footpad bounces into and out of contact with the work piece. Both regular and random vibrations are transmitted from the power tool to the hands of the operator who uses the tool. Prolonged exposure to vibration can result in discomfort to the operator, and government regulations in some countries place daily limits on operator exposure to a range of vibrational levels for workers who use handheld power tools.
Low-frequency random vibrations are known to have a larger overall effect on the operator of a hand-held tool. FIG. 12 depicts a weighting chart used with the ISO-5439 standard, which is a well-known standard related to quantifying the effect of vibrations on humans. The weighting factor identified by the line depicted in the weighting chart of FIG. 12 increases from an initial value at a vibration of 1 Hz to a maximum around a vibration frequency of 12.5 Hz. The weighting factor corresponds to the relative effect of vibration at a given frequency on the operator of the handheld power tool. As depicted in FIG. 12, the effects of low frequency vibrations, especially vibrations from about 1.75 Hz to about 250 Hz, are much greater than higher frequency vibrations. Consequently, a vibrational dampener that dampens low-frequency vibrations effectively can benefit an operator more than a high-frequency vibrational dampener.
Various devices and techniques are known to reduce the amount of vibration that an operator experiences when operating a handheld power tool. Some tools include passive vibrational dampeners that use rubber members and springs to reduce the level of vibration that an operator experiences as the tool operates. Passive vibrational dampeners can reduce the magnitude of vibrations that the operator experiences while using the tool, but the passive vibrational dampeners are generally not effective in dampening random vibrations. In particular, the passive vibrational dampeners have resonant frequencies that match low-frequency random vibrations of less than 250 Hertz, and medical data indicate that low-frequency vibrations are also responsible for a majority of the discomfort that an operator experiences while operating the power tool. When exposed to random vibrations near the resonant frequency of the passive dampener, the passive vibrational dampeners experience sympathetic vibration and can amplify the vibrational forces instead of dampening the vibration.
Other disadvantages of passive elastomeric vibrational dampeners include the sizes of the dampeners and the temperature response of the elastomeric materials. In a compact handheld tool, the volume of elastomeric material needed to effectively dampen vibration is often too large to fit inside the tool. Additionally, the vibration dampening coefficients of passive materials vary as the temperature of the power tool changes. In a “cold” state as the power tool begins operation, the elastomeric material may be too rigid to effectively dampen vibrations. As the temperature of the power tool increases during operation, the elastomeric material softens and may become too soft to effectively dampen vibration. Typical elastomeric materials are incapable of providing effective vibrational dampening over a wide range of temperatures.
Another type of vibrational dampening device includes semi-active vibrational dampeners. A semi-active vibrational dampener is a vibrational dampener that adjusts a damping coefficient of the vibrational dampener dynamically. A control circuit typically adjusts the dampening coefficient based on one or more vibrations in the handheld power tool. While semi-active dampening devices can have advantages over passive dampening devices, using a semi-active dampening device in a handheld power tool also presents various challenges. One challenge is that handheld power tools, such as reciprocating saws, typically have a limited amount of space to accommodate the vibrational dampener while also being small enough for an operator to hold comfortably. Existing power tool designs have placed a small semi-active dampener in a rear handle of the power tool. The design of the handle limits the size of the vibrational dampener, and even if the vibrational dampener is effective, the vibrational dampener only reduces vibration experienced at the rear handle. Frequently, operators place at least one hand in locations other than the rear handle. Consequently, the existing semi-active systems cannot effectively dampen vibration that is transmitted to both hands of the operator positioned at different positions on the tool.
As described above, both the passive and semi-active vibration dampening systems known to the art have drawbacks when incorporated into handheld tools. Consequently, improvements to the design of handheld tools that enable improved vibrational dampening for two-handed operation would be beneficial.